1. Field of the Invention
This invention relates to sample collection and more particularly relates to a method for coating a blood sample collection tube.
2. Background
Blood samples are routinely taken in evacuated tubes. One end of a double-ended needle is inserted into a patient""s vein. The other end of the needle then punctures a septum covering the open end of the tube so that the vacuum in the tube draws the blood sample through the needle into the tube. Using this technique, a plurality of samples can be taken using a single needle puncture of the skin.
Blood drawn into a tube is typically mixed with an additive present in the tube prior to draw. Clot activators such as silica particles promote rapid coagulation so that the liquid serum fraction can be readily separated from the clotted cells. Anticoagulants are used to prevent clotting when the blood sample is to be used directly in hematological tests or for separation of blood cells from the plasma. For separation, the blood sample is centrifuged, and a gel having a specific gravity between that of the solid and liquid fractions is used as a barrier between the fractions.
Collection tubes are conventionally made of glass or plastic, such as polyethylene terephthalate (PET). Glass tubes have the advantage of water and gas impermeability. Plastic tubes are advantageous over glass in lower breakage, less weight in shipment and easier disposal by incineration, but high permeability to water and gas is a disadvantage.
In glass it has been conventional to provide the additive in aqueous solution to provide rapid mixing with the blood. However, the action of a liquid additive in the tube may be masked by the gel. Also, if the additive is applied to the wall of the glass tube, it rapidly runs down the tube wall and collects in the bottom of the tube where, again, its action may be masked by the gel. For this reason the tube wall is often coated with polyvinylpyrrolidone (PVP) and the anticoagulant solution applied thereover. The PVP dissolves in the blood and becomes part of the plasma.
Aqueous additive formulations are unsatisfactory in plastic because moisture permeation through the tube wall changes additive concentration, may reduce additive functionality and may reduce shelf life. In one approach to providing additives in plastic tubes, a plastic blood collection tube contains a plastic film insert having an anticoagulant affixed thereto. Such tubes have the disadvantage that the film becomes part of the packed cells and can interfere with analyses which use the cells, such as blood typing.
In another approach, the inside wall of a plastic tube is spray coated with an ultrasonic coating device and the coating is air dried.
There is a need for a plastic blood collection tube having a uniform sprayed-on additive which is readily air dried to a solid form which will readily dissolve when blood enters the tube and yet leave both the plasma and cell fractions free of extraneous materials. The present invention fulfills this need.
A blood collection assembly includes a collection tube having a bottom wall continuous with a side wall. The side wall defines an open end and the bottom wall defines a closed end. Together the bottom and side walls define an inside wall surface. Preferably, the open end is covered by a puncturable stopper and the tube is evacuated. A conventional thixotropic gel may be stored in the bottom of the tube. The inside wall surface is coated with an additive for blood analysis applied by spraying a solvent dispersion of the additive through an air nozzle and drying to leave a layer of discrete particles. In this application, the term dispersion includes both a solution and a suspension.
A second aspect of the invention is a method to make the tube.
Use of an air nozzle instead of a conventional ultrasonic nozzle provides several advantages. The mist provided by an air nozzle is finer and more uniform, thereby eliminating coalescing of the spray into droplets and subsequent rundown of fluid, even on the inside wall of a glass tube. Avoiding rundown facilitates drying and gives uniform coatings of discrete solid particles of greater surface area. In addition air nozzles are much less complex than ultrasonic nozzles and therefore provide substantial cost savings and ease of adaptation for automated manufacturing lines.